The invention relates to a method for investigating the functioning of the lungs of a subject, and more particularly to measuring the real flow respiratory resistance of breathing passages.
Apparatuses for analysing the functioning of the lungs serve for the early or timely recognition of obstructive diseases of the lungs, and also for the recognition of restrictive respiratory disturbances. They also make it possible to carry out pulmonary provocation investigations and tests for bronchialitis.
It is known within the context of lung function analysis to determine as respiratory parameters the respiratory volumes, the air flow and the flow resistance. The respiratory volume represents a static respiratory parameter, the air flow a dynamic respiratory parameter and the flow resistance the resistance of the breathing passages, whereby the pressure difference between the alveoli and the environmental air which has to be overcome corresponds to the flow resistance of the breathing passages.
The determination of the respiratory volumes and of the air flow takes place in known manner either in closed or half-open systems, or in open systems. In the context of the present invention it is the so-called open system which is in particular of significance, in which the quantity of air flowing is detected in an open tube and the respiratory volumes are deduced therefrom.
The resistance of the respiratory passages is determined in the context of the present invention in accordance with the oscillation method in which small oscillations are impressed on the respiratory flow of the patient. Furthermore, the method of frequency modulated oscillation is of significance in the context of the invention. By means of this method the resonant frequency of the respiratory passages of the subject can be deduced by frequency modulation of the oscillation. In this connection it is of advantage that the resistance measured at the resonant frequency is identical With the real flow resistance of the respiratory passages, because other resistances such as inertia and extensibility of the lungs cancel out.
The object of the present invention is to provide a method and an apparatus for the diagnosis of the lung function which operates extremely reliably and with high accuracy, which enables the determination of all the parameters of interest, which can be realised in compact form, which ensures simple operation and which meets the highest requirements from the point of view of hygiene.
It is important for the solution of this problem that one operates, in accordance with the invention, according to the so-called oscillation method in conjunction with a differential pressure measurement and an absolute pressure measurement, and that one simultaneously uses a branch principle which makes it possible to operate without a specific precisely defined terminal resistance. By measuring the differential pressure, i.e. the difference of the pressures on the two sides of a fine-meshed wire net arranged in a measuring tube, by measuring the absolute pressure in the subject side of the measuring tube, and also by feeding a pulsed partial flow into the measuring tube at least on the side of the fine-meshed wire net remote from the subject, whilst simultaneously leading off the remainder of the partial flow via a hose section of non-critical length which continues the measuring tube, one obtains surprisingly high measurement accuracies with the simplest manner of operation which is most favourable for the subject.
A pulsed air supply is preferably used with a steady flow component such that a part of the pulsed air which is supplied continuously emerges via the hose section which belongs to the branch, which ensures that the subject breathes in exclusively freshly supplied air and that the sucking back of consumed air does not occur. This is of significance because in this way it is ensured that falsification of the measured values by re-breathed air can be avoided.
Furthermore, it is a significant advantage that constant measurement conditions are obtained, in particular with regard to temperature and humidity which again contributes to an increase of the accuracy of the finally obtained measured values.
Having regard to the attainment of a measurement accuracy which is as high as possible, it is furthermore of important significance for the invention that an element with a very low mass is used as the fine-meshed grid or net which can be rapidly controlled temperature-wise by a small supply of energy in order to prevent condensation effects on the grid or net. The grid or net is preferably inductively heated with the temperature being regulated in dependence on the flow, i.e. the quantity of air and the direction of the flow as well as the temperatures of the flows in the respective directions are taken into account in the computer controlled regulation in accordance with the invention. The mean temperature of the net or grid is moreover directly detected via a sensor.
In its practical realisation a system in accordance with the invention consists of a basic apparatus with an integrated printer, an input unit which is in particular constructed as a foil keyboard, a measuring head with pneumotachographs for measuring volumes and flow, and also a pressure pick-up part for measuring the resistances and a video screen for presenting data and graphs. The flow curve and the volume curve are preferably plotted relative to the time axis and the flow curve relative to the volume is preferably also shown. Moreover, the actual air pressure and also the room temperature are independently deduced and made use of during the running of the program.
It is possible, as a result of the measured values that are obtained to derive all the important lung function parameters and indeed in particular the vital capacity, the inspiratory reserve volume, the expiratory reserve volume, the breathing volume, the forced vital capacity, the absolute second capacity and also the relative second capacity. The real actual values that are deduced can be set into relation with the stored normal values which represents a significant simplification so far as the evaluation is concerned.
Both the inspiratory and also the exspiratory flow volume curves can be determined by means of a microprocessor controlled computer from the volumes measured in the pneumotachograph in relation to the time axis, with the maximum exspiratory flow, the maximum inspiratory flow and also the forced exspiratory flow and further relative values being additionally derivable.
Further special features of the invention will be described in the following with reference to the drawing.